Recently, for steel materials for welded structure and the line pipe for mainly transporting the crude oil or the natural gas, in addition to high strength and high toughness, low yield ration is required in the light of earthquake-proof. Generally, it is known that a metal structure of a steel material is formed into a structure in which a hard phase such as bainite or martensite is appropriately dispersed in a soft phase such as ferrite, thereby the low yield ratio of the steel material can be achieved.
As a manufacturing method for obtaining the structure in which the hard phase is appropriately dispersed in the soft phase as above, a heat treatment method where quenching (Q′) from a two-phase range of ferrite and austenite ((γ+α) temperature range) is performed between quenching (Q) and tempering (T) is known (for example, see JP-A-55-97425). In the heat treatment method, the low yield ratio can be achieved by appropriately selecting the Q′ temperature, however, since the number of heat treatment steps increases, reduction in productivity and increase in production cost are caused.
As a method without increasing the number of manufacturing steps, a method is disclosed, in which after rolling has been finished at Ar3 temperature or more, start of accelerated cooling is retarded until the steel material is cooled to the Ar3 transformation point or lower where ferrite formation occurs (for example, see JP-A-55-41927). However, since cooling needs to be performed at a cooling rate of roughly standing to cool in a range from rolling finish to accelerated cooling start, productivity is extremely lowered.
In the welded steel pipe such as UOE steel pipe or electric welded tube used for the line pipe, since a steel plate is formed into a tubular form in cold working, and then abutting surfaces are welded to each other, and then typically coating treatment such as polyethylene coating or powder epoxy coating is applied on an outer surface of the steel pipe in the light of anticorrosion, strain aging occurs due to work strain during pipe production and heating during the coating treatment, thereby yield stress increases. Therefore, even if low yield ratio is achieved in the steel plate as material in the method as above, low yield ratio is hard to be achieved in the steel pipe.
As a steel material having excellent strain aging resistance and a method for manufacturing the material, a method is disclosed, in which content of C and N that cause the strain aging is limited, in addition, Nb and Ti are added and combined with C or N, thereby the strain aging is suppressed (for example, see JP-A-2002-220634).
However, in the technique described in JP-A-2002-220634, as shown in an embodiment of it, since hot rolling finish temperature is low, productivity is extremely lowered, resulting in increase in production cost.
As a technique for achieving the low yield ratio without performing the complicated heat treatment as disclosed in JP-A-55-97425 and JP-A-55-41927, a method is known, in which rolling of a steel material is finished at an Ar3 transformation point or more, and a rate of subsequent accelerated cooling and cooling stop temperature are controlled, thereby a two-phase structure of acicular ferrite and martensite is formed, and thereby the low yield ratio is achieved (for example, see JP-A-1-176027).
However, in the technique described in JP-A-1-176027, as shown in an embodiment of it, since carbon content in the steel material needs to be increased, or other alloy elements need to be added more so that a steel material of tensile strength of 590 N/mm2 (60 kg/mm2) class is formed, deterioration of toughness of a welding heat affected zone is problematic in addition to increase in material cost.
In this way, in the related arts, it is difficult to manufacture the steel pipe having the low yield ratio after coating treatment without reducing productivity, without increasing the material cost, without degrading toughness of the welding heat affected zone, without lowering productivity of the low yield ratio, high strength and high toughness steel plate or steel pipe, and without increasing production cost of the steel pipe.
International Publication WO03/006699 A1, which is a technique previously developed by the inventors of the application, is an invention on a high-strength welded steel pipe having excellent HIC resistance or post-welding toughness by forming a single phase of ferrite in which a complex carbide is finely precipitated. However, since island martensite does not exist in the structure unlike this application, the steel plate having a low yield ratio as an object of the application can not be obtained.